The contributions of layering and sequential ventilation will be evaluated quantitatively by using the fact that after airway closure expiration is almost entirely sequential. If expiration is sequential the ratio of differences in concentration of any two insoluble gases between the point of airway closure and any point later in expiration will be a constant. If one of the gases is introduced as a bolus early in inspiration and is known to be ventilated only sequentially this constant ratio can be applied to the early part of the expiratory curve of the second gas to determine which portion is sequential and which part due to layering. A method is being developed to calculate regional pulmonary diffusing capacity by measuring arterial O2 saturation at various inspired O2 tensions using an analog computer to simulate a 2 compartment lung having the volumes and ventilation determined on a previous N2 washout. It is believed that a unique value for the V/Q ratio of the poorly ventilated space and for the Bohn integral of this space can be obtained by this simulation.